Method of dimensioning and operating a low pressure discharge lamp

ABSTRACT

In a method of dimensioning and operating by A.C. or D.C. at a predetermined external heater power a low pressure discharge lamp, particularly a fluorescent lamp having two electrodes between which the discharge is formed, at least one of the electrodes being alkaline earth oxide coated and adapted to form a permanently heated cathode, particularly for use in video matrix board, and external heater current is used which is approximately 1.5 to 5 times higher than the discharge current, the heater volt is approximately 33.33% to 80% lower than that of a conventionally operated cathode or electrode, respectively, and the wattage of the heater circuit is maintained. By this accuracy of optimum cathode temperature control is improved, enhanced lamp life and reduced discoloration are obtained and the formation of hot spots avoided.

FIELD OF THE INVENTION

The invention relates to a method of dimensioning and operating by A.C.or D.C. at a predetermined external heater power a low pressuredischarge lamp having two electrodes between which the discharge isformed, at least one of the electrodes being alkaline earth oxide coatedand adapted to form a permanently heated cathode, particularly for usein video matrix displays.

BACKGROUND OF THE INVENTION

In low pressure discharge lamps the discharge is formed betweenelectrodes. These operate alternatively as anode or cathode,respectively, if the lamp is supplied with A.C., or permanently as anodeor cathode in case of D.C. operation. Cathodes consist usually of acoiled tungsten wire which is coated with a mixture of alkaline earthoxides to enhance thermionic electron emission.

The life of a fluorescent lamp is mainly determined by the life of thecathode.

The physical behaviour of an oxide coated cathode is complex. Roughlythe cathode exists in a sensitive equilibrium of thermionic emission andevaporation of emissive material.

SUMMARY OF THE INVENTION

There is an optimum temperature of the emissive coating at whichelectron emission is high enough to maintain the discharge andevaporation is low enough to grant sufficient life.

Rapid starting of a low pressure discharge lamp is accomplished bycurrent heating of the cathode--or, in case of A.C. operation, bothelectrodes--to a temperature which provides for sufficient thermionicemission. The heating mode is permanent, i.e. the heater is externallyand permanently heated. The external heater current is normally chosenequal to the discharge current.

In operating low pressure discharge lamps, particularly fluorescentlamps with externally heated cathode(s) the discharge current issuperposed to the heater current and therefore forms a locallyoverheated area (hot spot). Due to temperature difference of about400-500 K encountered with the known lamps and their operation asdescribed above, the evaporation rate of the emissive oxides increasesby orders of magnitude. This, in turn, leads to increased blackeningand, ultimately, reduced life of the lamp.

It is an object of the present invention to provide a method asmentioned above in the first paragraph of the description by which thefunctional life of lamps as indicated above can be increased and the endblackening or end discoloration, respectively, reduced.

This object is met in that an external heater current is used which isapprox. 1.5 to 5 times higher than the discharge current and that theheater voltage is approx. 33.33% to 80% lower than that of aconventionally operated cathode or electrode, resp., and that thewattage of the heater circuit is maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a low pressure discharge lamp having two cathodes;

FIG. 2 is a photograph showing a cathode having a hot spot; and

FIG. 3 is a photograph showing a cathode having no hot spot.

BEST MODE FOR CARRYING OUT THE INVENTION

With particular attention to FIG. 1, there is illustrated a low pressuredischarge lamp 10 having a U-shaped tube and two electrodes 14 and 16.

The main advantage of the inventive method resides in the fact that toimprove the accuracy of control of the desired optimum cathodetemperature and thereby enhance lamp life and reduce discoloration thecontribution of discharge current heating is minimized and thecontribution of the external heater current is maximized so that cathodetemperature will not constantly rise and fall to such extents that theadverse effects elucidated above including the formation of hot spotsare encountered.

The inventive dimensioning of the external heater current on one sideand the heater voltage on the other will normally include to increasecathode wire size and/or to decrease total filament length to permitcoil heat to be supplied at higher current and lower voltage.

A preferred embodiment of the method according to the invention ischaracterized in that for increasing the heater current I to n·I anddecreasing the heater voltage U to U/n the radius r of the wire formingthe cathode or electrode, resp., is dimensioned to √n·r, the crosssection A of the wire is increased to n·A and the wire length L isreduced to L/n, whereby the resistance R is reduced to ##EQU1##

Preferably, n is a number equal to approximately 1.5 to 5.

The reduced resistance of the coll reduces the I² ·R heating by arccurrent. The higher heat conductivity of the heavier wire moreeffectively dissipates ion bombardment heating in the cathode spot, withreduced temperature rise. Thus, the cathode spot temperature becomesmuch less sensitive to arc current conditions and is controlled by thecoil heat power instead.

Achieving reliable uniform life of fluorescent lamps requires achievingreliable control of cathode operating temperature. This is possible forthe first time by means of the instant invention. If cathodes aredesigned in accordance with the prior art, with a substantial portion ofthe cathode power input being derived from the arc current, it isapparent that satisfactory control of cathode spot temperature overdifferent discharge current conditions will be impossible to achieve.This holds good particularly for fluorescent lamps used in video matrixboards where operation of each lamp may occur at anyone of e.g. 32different discharge currents on duty cycles ranging from continuousburning at maximum current to being excited only for a few hundredths ofa second at a low current.

The invention has been verified in practice with an embodiment in whichthere was used an external heater current being 3.2 times higher and aheater voltage being 3.2 times lower than that with a conventionaloperated cathode. Reference is made to FIG. 2 and photo II as attached,FIG. 2 showing a cathode having a hot spot of 450° K. over temperature,whereas FIG. 3 is showing a cathode having no hot spot and an optimalthermionic emission temperature.

If the cathode in accordance with FIG. 2 is called a type I and thecathode in accordance with FIG. 3 is called type II, the following is anindication of the differences between both types, type II beingidentical to type I except for heater voltage, heater current and heateroperation resistance, please see the following data:

Type I

U-shaped tube

One electrode: oxide coated tungsten coil cathode

One electrode: anode

gas fill: argon, mercury

fill pressure 3.5 mbar

without phosphor

discharge current 125 mA D.C.

arc voltage: 25 V

heater voltage: 8 VD.C.

heater current: 0.125 A D.C.

heater operation resistance: 64 Ohm

heater power: 1 W

Type II

identically, except of:

heater voltage: 2.5 V D.C.

heater current: 0.4 A D.C.

heater operation resistance: 6.25 Ohm

heater power: 1 W

Measured thermionic emission temperature:

Type I: 1350° C. (hot spot)

Type II: 900° C. (without hot spot)

As a conclusion, if the external heater current of a permanently heatedfluorescent lamp is substantially higher than the discharge current,local overheating of the cathode or the formation of a hot spot, resp.,is reduced or eliminated and results in increased lamp life and lessblackening.

Particularly in case of intensity modulated fluorescent lamps asrepresented by video matrix display lamps used in the boards mentionedabove it is possible to supply the cathode with A.C. instead of D.C. Inaddition, the vibration resistance of the lamps is improved.

We claim:
 1. Method of dimensioning and operating by A.C. or D.C. at apredetermined external heater power a low pressure discharge lamp,particularly a fluorescent lamp having two electrodes between which thedischarge is formed, at least one of the electrodes being alkaline earthoxide coated and adapted to form a permanently heated cathode,characterized in that an external heater current is used which isapproximately 1.5 to 5 times higher than the discharge current, that theheater voltage is approximately 33.33% to 80% lower than that of aconventionally operated cathode or electrode, respectively, and that thewattage of the heater circuit is maintained.
 2. Method according toclaim 1, characterized in that for increasing the heater current I ton·I, wherein n is a number, and decreasing the heater voltage U to##EQU2## the radius r of the wire forming the cathode or electrode,resp., is dimensioned to √n·r, the cross section A of the wire isincreased to n·A and the wire length L is reduced ##EQU3## whereby theresistance R is reduced to ##EQU4##
 3. Method according to claim 1 or 2,characterized in that there is used an external-heater current which is3.2 times higher and a heater voltage which is 3.2 times lower than thatwith a conventionally operated cathode or electrode, respectively.